One of the major problems associated with the development and use of internal combustion engines is the noxious exhaust emissions from such engines. Two of the most deleterious materials, particularly in the case of diesel engines, are particulate matter (primarily carbon) and oxides of nitrogen such as nitric oxide NO and nitrogen dioxide NO2 often referred to as (NOx).
Excessive levels of NOx also are produced by spark-ignition engines operating in what is known as ‘lean burn’ mode in which the air/fuel (e.g. gasoline) ratio is higher than that required for stoichiometric combustion. It is also appreciated that alternative fuels and hybrid type combustion engines, as an example which may burn diesel fuel and/or natural gas, may also pose a similar problem. Increasingly severe emission control regulations are forcing internal combustion engine and vehicle manufacturers to find more efficient ways of removing these materials in particular from internal combustion engine exhaust emissions.
One of the ways in which emissions are being reduced is by modifying the combustion process in the engine. Modifications include altering injection timing, engine design, common rail systems and exhaust gas recycling but all have certain limits for practical engine operation. Unfortunately, in practice, it is often found that techniques which modify the combustion process to improve the situation in relation to one of the above components of internal combustion engine exhaust emissions can tend to worsen the situation in relation to the other.
There are numerous techniques being developed to remove NOx emissions from exhaust gases from internal combustion engine exhaust as well as other waste gas sources. In general, practical NOx reduction systems for internal combustion engines are reliant on passing the exhaust gases across a catalyst. There are generally two types of catalytic reduction methods used, non-selective and selective catalytic reduction (SCR). This invention is concerned primarily with SCR systems and requires a suitable reductant or reducing agent to be present or added to the exhaust gas. Typical reductants for this purpose are urea or ammonia, but these are not the most practical for mobile vehicle applications. This is because this needs additional space for the reductant tank on the vehicle and a supply infrastructure to allow the reductant to be replenished. SCR catalysts can however perform very effectively using hydrocarbons, normally found in the combustion engine exhaust, as the reductant for a certain range of temperatures. One of the key issues with this approach is whether the exhaust gas has the required concentration of hydrocarbon reductant present to promote the required selective catalytic reactions to reduce NOx to nitrogen. The concentration of hydrocarbons may be altered, if there is insufficient in the exhaust, by for example, adding a post-injection of fuel into the combustion chamber or by injecting fuel into the exhaust. One recently developed method is to use non-thermal plasma to activate the hydrocarbon, which may be in the form of additional fuel, to promote the catalytic NOx reduction to nitrogen as disclosed in WO99/12638.
Considerable effort has been dedicated to the development of catalysts for the reduction of NOx from diesel exhausts. The paper ‘Selective Catalytic Reduction of NOx with N-Free Reductants’ by M. Shelef published in Chem. Rev. 1995 pages 209–225 is a comprehensive review in particular of the use of zeolites for the reduction of the NOx content of internal combustion engine exhaust gases. Other catalysts are mentioned but not dealt with comprehensively. The more recent review by Parvulescu et al ‘Catalytic Removal of NO’ published in Catalysis Today, volume 46 (1998) pp 233–316 is a comprehensive document on the range of materials that have been evaluated for the selective catalytic reduction of NOx. This is NOx reduction using catalysts such as zeolites, including metal exchanged zeolites, oxides such as simple oxides, for example Al2O3, V2O5, complex oxides, such as perovskites, and precious metal supported oxides, in the presence of reducing agents such as hydrocarbons or ammonia. All of the materials described in this review are used solely as thermally active catalysts.
U.S. Pat. No. 5,149,511 discloses a system for reducing NOx emissions from oxygen-rich internal combustion engine exhaust gases in which partially oxygenated light organic compounds such as alcohols, aldehydes, ketones and ethers, are injected into the exhaust gases which are then passed over a bed of a copper-containing ZSM5 zeolite and then a bed of an oxidising catalyst such as Pt doped alumina or 1% Pd/10% La2O3/Al2O3 to remove any unreacted reductant.
Despite extensive world-wide efforts it has been difficult to find an effective catalyst for selective catalytic reduction of NOx because candidate materials can be deactivated in use, for example by water vapour at typical diesel exhaust temperatures. The selectivity of the catalyst is difficult to control, as the optimum operating temperature of the catalyst does not always coincide with the exhaust gas temperature. In practice, the catalyst may not be wholly selective to NOx, for example it may oxidise hydrocarbon present in the exhaust gases at the expense of the selective catalytic reduction of NOx to N2. There is also considerable concern that the current selectivity of SCR catalysts operating in lean engine exhausts (Lean-NOx catalysts) is poor. This means that undesirable species such as N2O are formed, which has a very strong greenhouse gas effect, instead of nitrogen (N2). Other selectivity problems include an apparent reduction of, for example NO2 concentration which is actually an inter-conversion back to NO, not a reduction to N2. A considerable number of catalysts are also reported to be reliant upon the NOx emissions to be predominantly NO2 for reduction to N2.
Multi-stage systems for the selective catalytic reduction of NOx have also been developed: U.S. Pat. No. 4,902,487 and the article by Cooper and Thoss ‘Role of NO in Diesel Particulate Emission Control’ published as SAE 890404, 1989 describe a two-stage system in which diesel exhaust is passed over a Platinum (Pt) oxidation catalyst, which oxidises NO in the exhaust gas. to NO2 after which NO2 reacts with carbonaceous particulates in the exhaust stream that are trapped on a filter. The NO2 effectively combusts the deposited carbon particulates and is thus reduced and products of this reaction are NO, N2, CO and CO2. A combustion catalyst for example lanthanum, caesium and vanadium pentoxide on the filter is used to lower the combustion. temperature of the carbon/NO2 reaction to around 538 Kelvin.
Iwamato et al in the article ‘Oxidation of NO to NO2 on a Pt-MFI Zeolite and Subsequent Reduction of NOx by C2H4 on an In-MFI zeolite: a novel de-NOx strategy in excess oxygen’ published in Chemical Communications pages 37–38, 1997, describe the use of a two-stage system whereby NO is first oxidised to NO2 by a Pt-containing MFI zeolite oxidation catalyst with maximum conversion at 423 K. Hydrocarbon, C2H4, is added to the oxidised gas stream which is passed over an In-containing MFI zeolite catalyst, whereupon the selective catalytic reduction of NO2 to N2 takes place in the presence of excess oxygen. PCT application WO98/09699 discloses an arrangement in which oxygen-rich exhaust gases are passed through a bed of an oxidising catalyst such as platinum-doped alumina in which NOx in the exhaust gases is oxidised to NO2. Hydrocarbons are mixed with the effluent from the oxidiser and the mixture is passed through a bed of a reducing catalyst, γ-alumina in which the NO2 and hydrocarbons are reduced to N2, CO2 and H2O.
Multi-stage systems using a combination of a non-thermal plasma and a catalyst for the treatment of NOx components of diesel exhausts have been proposed.
GB Patent Application 2,270,013 A describes a two-stage system in which exhaust emissions from internal combustion engines are subject to a low temperature plasma and then passed over a catalyst that is downstream of the plasma. Although not specifically mentioned in GB 2,270,013 A it will be appreciated that the exhaust emissions can contain nitrogen oxides.
U.S. Pat. No. 5,711,147 describes a two-stage system in which a non-thermal plasma oxidises NO in a gas stream to NO2 and the latter then undergoes selective catalytic reduction to N2 in the presence of C3H6 over a γ-Al2O3 catalyst. The system is for use with oxygen-rich exhaust gases from diesel and lean-burn spark ignition engines. In the system described in U.S. Pat. No. 5,711,147 a hydrocarbon such as diesel fuel is cracked into simpler hydrocarbons by a corona discharge and then is mixed with the oxygen-rich exhaust gases from which NOx is to be removed. The mixed hydrocarbons and exhaust gases are then passed through another region of corona discharge, which may include silica beads as a particulate trap. In this region, NOx is oxidised to NO2. The NO2 plus excess hydrocarbons are passed through a bed of a catalyst which acts to reduce the NO2 to O2 and N2 and to oxidise the hydrocarbons to CO2 and H2O. No plasma is involved in the reduction stage. There is a requirement for the pre-conversion of NO to NO2 before selective catalytic reduction in U.S. Pat. No. 5,711,147 as the catalyst used is more efficient for the reduction of NO2 reduction than for the reduction of NO. In addition sufficient hydrocarbons have to be present to enhance the plasma oxidation of NO to NO2 and to act as a reductant for reduction of NO2 to N2.
WO00/18494 describes a method and apparatus in which a gas stream containing NO and hydrocarbon is passed through a plasma and then over a catalyst comprising a microporous material, particularly a zeolite, resulting in the reduction of Nox to nitrogen. Results shown in WO00/18494 indicate that the percentage NOx reduction was as high as 77%, but it could be as low as 4% depending on the catalyst used for temperatures in the range 373–573 K.
GB patent 2,274,412 discloses a method and apparatus for removing particulate and other pollutants from internal combustion engine exhaust gases. In addition to removing particulates by electric discharge assisted oxidation such as by use of a non-thermal plasma, there is disclosed the reduction of NOx gases to nitrogen, by the use of a bed of pellets adapted to catalyse the NOx reduction.
Also, U.S. Pat. Nos. 3,983,021, 5,147,516 and 5,284,556 disclose the catalytic reduction of nitrogen oxides. However, U.S. Pat. No. 3,983,021 is solely concerned with the reduction of NO to N in a silent glow discharge, the temperature of which is kept below a value at which the oxidation of N or NO to higher oxides of nitrogen does not occur.
Although, so-called contact bodies are used in the process of U.S. Pat. No. 3,983,021, and some of those disclosed may have some catalytic properties, catalysis does not appear to be a necessary feature of the process of U.S. Pat. No. 3,983,021. Other surface properties, such as adsorption on large surface area materials, are the basis of the process of U.S. Pat. No. 3,983,021.
U.S. Pat. No. 5,147,516 does refer to the use of catalysts to remove NOx, but the catalytic materials involved are defined as deriving their catalytic activity from their form rather than their surface properties.
Also, the operating conditions are very tightly defined. There is no specific mention of the type, if any, of electric discharge involved. All that is disclosed is that the NOx removal depends upon electron-molecule interactions, facilitated by the structure of the ‘corona-catalytic’ materials.
PCT specification WO99/12638 describes a method for the treatment of internal combustion exhaust gases in which nitrogen oxides are removed by a process which includes the operations of passing hydrocarbons through a plasma in which there is a first material having oxidative properties in the presence of a plasma thereby to produce plasma activated hydrocarbons and contacting a mixture of the activated hydrocarbons and the exhaust gases with a second material adapted in the presence of the activated hydrocarbons to catalyse the reduction of the nitrogen oxides to nitrogen.
Among the materials for carrying out the second step of the invention disclosed in WO99/12638 are various forms of alumina including activated alumina. Activated aluminas include the γ-alumina phase. Such materials also are disclosed in many of the other examples of prior art mentioned above.
It has been found that, in practice, γ-aluminas in particular have the disadvantage that their reactivities are sensitive to the presence of water and as internal combustion engine exhaust gases include amounts of water vapour which vary with the operating conditions of the engine concerned, the performances of the catalysts are variable.
Silver-based catalysts have been described for the reduction of NOx in vehicle emissions. In the papers by Miyadera “Alumina-supported silver catalysts for the selective reduction of nitric oxide with propene and oxygen-containing organic compounds” published in Applied Catalysis B: Environmental, volume 2, (1993) pages 199–205, and Miyadera and Yoshida “Alumina-supported silver catalysts for the selective reduction of nitric oxide with propene” published in Chemistry Letters, (1993), page 1483 a 2% Ag-alumina catalyst showed promising hydrothermal stability for NOx reduction. Added propene and partially oxygenated hydrocarbons, such as 2-propanol, were effective reductants. Masuda et al in the article “Silver promoted catalyst for removal of nitrogen oxides from emissions of diesel engines” in Applied Catalysis B: Environmental, volume 8, (1996), pages 33–40 showed that 3% Ag-mordenite was a promising lean NOx catalyst compared to Ag-ZSM-5 and Ag-alumina with CH3COCH3 as reductant. Bethke and Kung in the paper “Supported Ag catalysts for the lean reduction of NO with C3H6” published in Journal of Catalysis, volume 172, (1997), page 93 showed that the oxidation state of silver affects its catalytic activity for the reduction of NOx. Another silver containing compound, silver aluminate, AgAl2O4, doped with 0.1 weight % WO3 was shown to be a promising catalyst for the reduction of NOx by Nakatsuji et al in the paper “Catalytic reduction system of NOx in exhaust gases from diesel engines with secondary fuel injection” published in Applied Catalysis B: Environmental, volume 17, (1998), pages 333–345. Keshavaraja et al in an article ‘Selective catalytic reduction of NO with methane over Ag-alumina catalysts’ published in Applied Catalysis B:Environmental, volume 27, pages L1–L9, 2000 used CH4 for the selective reduction of NO over silver-alumina catalysts at temperatures between 723–923 K with Ag loadings of 1–7 weight percent.
Meunier et al have discussed the role of silver-alumina catalysts on the selective catalytic reduction of NO by propene in an article ‘Mechanistic aspects of the selective reduction of NO by propene over γ-alumina and silver-alumina catalysts’ published in Journal of Catalysis, volume 187, pages 493–505, 1999. High silver loadings, 10 percent by weight produced N2O while a low loading, 1.2 percent by weight,.was effective for the selective catalytic reduction of NO to N2. Adsorbed organo-nitrogen compounds such as organo-nitrites were intermediate species in the reaction.
Masters and Chadwick showed that oxygenated hydrocarbons, methanol and dimethyl ether can reduce NO to N2 under lean conditions by selective catalytic reduction over γ-alumina. This work, ‘Selective reduction of nitric oxide by methanol and dimethyl ether over promoted alumina catalysts in excess oxygen’, published in Applied Catalysis B: Environmental, volume 23, pages 235–246, 1999 showed that molybdena (MoO3) additions improved the catalytic activity at temperatures lower than those required in the case of γ-Al2O3 alone. Surface formyl species were an intermediate product in the reaction.
However, the above published work on silver-, or molybdena (MoO3)—, containing catalysts has been carried out in circumstances which have not involved the use of non-thermal plasmas. Silver-, or molybdena (MoO3), containing alumina based catalytic materials have not been proposed for use in the plasma assisted catalytic treatment or internal combustion engine exhaust emissions, The operation of catalytic materials in a plasma-assisted process environment is often different to that with operation in the absence of plasma, and is not straightforwardly predictable. Not only does direct exposure to the plasma affect catalytic performance in a number of ways, but also species formed in the plasma of a plasma-assisted process can affect the activity of catalytic materials whether or not the catalytic material is subjected directly to the plasma. The plasma can also enhance or even promote a catalytic effect on materials which are either slightly catalytic or show no catalytic behaviour at all.